Problem:
The drains were not working correctly, I guessed, because the level got almost to the point of overflowing the emergency drain.

Note the 2/3 weir.

So I (thanks to my wife's suggestion) limited the flow from one of the pumps.

A step in the right direction, for the wrong reasons.

Solutions:
First, I 'submerged' the drains, just by adding a coupling that extended the piping to just below the water level. You will note I also have my coarse filter in place. Such a cost...ouch.

Second, I made the 'adjusted' drain completely air-tight (supposed to be in the design anyway). Yes, that is a latex glove zip-tied to the top of the drain.

New water level:

Full weir:
Forgot to mention...I levelled the tank during all this (well, the stand), so now a nice full weir is present!

Summary of what I learned:

The full-siphon drain pipe must be completely air-tight - 100%, not 99%

The tank must be level for the weir to function properly

Duct tape functions as a ghetto bulkhead

A functioning coast-to-coast w. BeanAnimal is totally worth the effort.

A note on adjusting the full-siphon drain...not sure why people are suggesting gate valves over ball valves, works great as designed. Anyway, if I crack it open one more 'tick', the entire overflow drains in seconds - at this point it's stable and only a dribble is going down the secondary drain.

I tested by unplugging the pumps - overflow drained in seconds, then just the trickle from the duct tape bulkhead coming in. Plugged them back in, and things had stabilized (full siphon) within 30 seconds.